Guo Ni

Analyses on vegetation index variation and its formation causes in oases in northwest China in recent 22 years.

This paper analyzes the variation of Normalized Difference Vegetation Index(NDVI) in spring,summer and autumn and the area change of some typical oases in the Yellow River Basin,the Hexi Corridor and Xinjiang Uygur Autonomous Region in recent 22 years using the monthly datasets of NDVI during the period of 1982- 2003,compares the annual change characteristics of NDVI in 3 decades,and researches the relations between the NDVI,climate and human activities in the oases.The results are as follows:(1) The NDVIs of all the oases were increased significantly in autumn,especially in summer in recent 22 years.Regionally,the increasing rate of NDVI was the highest in the Shihezi Oasis(0.0075/a),and that of the oases in the Hexi Corridor and the southern marginal zone of the Tarim Basin were higher than that of others oases;(2) The variations of NDVI of all the oases in different decades were obvious,especially in the increase of NDVI in plant growth season and the change of occurring time of the peak value of NDVI;(3) The areas of the oases in northwest China were all enlarged in recent 22 years,especially in the Hexi Corridor and Xinjiang;(4) The NDVI increase was mainly caused by the enlargement of oases and the regulation of farming structure,and both temperature increase and precipitation increase had positive effects on the NDVI increase.

Quantitative identification dust and sand storm using MODIS data

Using MODIS data, the spectrum characteristic of sand and dust storm was analyzed in detail; bands that can distinguish sand and dust from cloud and surface were detected; two indices for determining the scope and intensity of sand and dust storm are designed, and were tested in several storms. Our study shows: 1) the spectral characteristic of sand and dust in solar wavelengths is that the reflectance increases along with the increasing of the wavelength. This is similar to the characteristic of the spectrum of soil. Also, the reflectance of large size dusts increases faster than small size dusts. 2) small size dusts show typical characteristic of aerosol, being sensitive to blue band of 0.46μm and insensitive to short-wave infrared bands of 1.6 and 2.1μm .3) large size dusts do not have aerosol characteristic, not sensitive to blue band and sensitive to short-wave infrared bands 4) 3.7μm and 8.5μm are sensitive to dusts. The difference of the two bands can be used to identify sands and dusts and reflect the intensity to certain extent. 5) The two indices are very effective to monitor sands and dusts. Also the method is simple and easy for operational use.